1. Field of the Invention
This disclosure pertains to a method of bonding solid materials. More particularly, it relates to bonding different materials with a method that includes formation of a micro-column array (MCA) on at least one surface.
2. Description of Related Art
Formation of Micro-Column Arrays (MCA) by laser processing of a silicon target irradiated with an excimer laser was reported several years ago. MCA consist of densely packed micro-cones, also called micro-columns, separated by cone-shaped cavities. These results have been duplicated in reactive and neutral atmospheres using a large variety of pulsed laser sources. The general feature of these structures is their protrusion above the initial surface of the sample by 10-20 μm. However, under exposure of silicon to radiation from a femtosecond laser the micro-cones are formed by etching the Si wafer and the micro-columns—i.e., material not removed by etching—are actually situated below level of the initial surface.
One of the areas that can benefit from employment of the MCA technology is bonding or joining of various solid materials. Bonding processes, also referred to as joining, are essential for large-scale manufacturing of components that employ titanium and aluminum base alloys, steels, ceramics, and composite structures. In general, bonding processes for similar metals include welding, brazing, and solid-state diffusion bonding. Bonding of dissimilar materials is much more complicated, especially at elevated temperatures, because compensation for the difference in the coefficients of thermal expansion (CTE) is required. Also, changes in chemical and structural properties that may occur during the thermally induced stress must be considered.
One of the most important parameters determining the mechanical properties of an interface is the ideal work of adhesion, Wad, which is defined as the bond energy needed (per unit area) to reversibly separate an interface into two free surfaces, neglecting plastic and diffusional degrees of freedom. Formally, Wad can be defined in terms of either the surface and interfacial energies (relative to the respective bulk materials), or by the difference in total energy between the interface and its isolated slabs:Wad=σ1v+σ2v−σ12=(E1tot+E2tot+E12tot)/As 
Here, σiv, is the surface energy of slab i, σ12 is the total interface energy, Eitot is the total surface energy of slab i, and E12tot to is the total energy of the interface system. As represents the total specific interface area. The total energy E=Wad×As required to separate layers will be proportional to this area, which may be greater than the area of a flat layer.
Although not predicted by classical models, an increase in flow stress is seen during deformation when the observed phenomenon is on the order of a micron and inhomogeneities are present. For example, Fleck et al. showed that when loaded in torsion, a wire displays greater strength for smaller radii (Fleck, G. M. Muller, A M. F. Ashby, and J. W. Hutchinson, Acta Metallurgical Materials 42, 475(1993)). Other authors have observed this type of effect in other systems, including bending, indentation hardness, and particle hardened alloys.
Employment of Micro-Column Arrays (MCA) can improve bonding between any solid materials performed by using adhesive or specialty brazing by not only increasing the area of the interface and reducing the stress due to the synergetic effect of the micro-deformations of single micro-cones, but also by promoting better wetting of the layer surface by the adhesive or the braze material due to the highly developed surface morphology. Thus employment of MCA for bonding or joining dissimilar components can significantly improve the bonding quality. “Bonding” as used herein refers to any process by which two materials are attached to one another by using a third material between them referred to as “bonding material”—e.g., melting brazing material or placing an adhesive between the materials to be bonded. For purposes of this disclosure, “bonding” and “joining” are synonymous and may be used interchangeably. “Bonding material” refers to anything placed between the two materials to be bonded that becomes part of the final bond. It includes, but is not limited to, various adhesives (e.g., epoxies), brazing, sintering materials, and solder. In the case of braze, for some applications it may be desirable to place a prebraze on a surface prior to applying the braze. Thus, prebraze is also a “bonding material” as the term is used in this disclosure.
Employment of epoxies and active brazing are two common methods currently used for solid material bonding. In many applications, mechanical or epoxy bonding limit design flexibility, have an undesirable appearance, and/or lower reliability due to poor epoxy strengths. Thus, complex and expensive metallic joining compounds that can wet and bond a variety of very difficult to join materials are used.
Brazing is widely accepted as a viable method for joining metals and ceramics. Brazing technology is particularly important in applications where metal ceramic composites cannot be used. These applications involve joining of highly dissimilar materials and the resulting joint has to sustain very high thermal and mechanical stresses. One example of conventional active metal brazing between titanium and fluorosilicate machinable ceramic-glass is given by using a 64 Ag-34.5 Cu-1.5 Ti (wt. %) brazing alloy. The reaction between the braze alloy and both materials at a temperature of 830° C. in vacuum leads to formation of multilayered interfaces with an average shear strength of more than 85% of the glass-ceramic bulk strength. A metal-ceramic composite substitute has been produced by joining a metal plate to a ceramic substrate by using a brazing material in a paste form comprising Ag—Cu-Active Metal-TiO powder. 97(Ag28Cu)3Ti and 97(Ag40Cu)3Ti (wt. %) braze alloys exhibited good performance for brazing of Al2O3 in vacuum at temperatures up to 1200° C. Brazing alloys for joining metal and ceramic plates can also contain 70-75 wt. % Ni, 45-55% wt. % Co, and 0.5-5 wt. % Ti.
There have been several approaches using conventional reflow soldering for bonding between ceramic and metal components; however, most of the approaches are either too complex, difficult to assemble, or ineffective.
Most ceramics, for example, are joined to other components using adhesives, fasteners, or specialty brazes. The latter method, specialty brazing, produces the strongest joints. They are also relatively better able to withstand high-temperature applications. Braze is sandwiched between the metallic and ceramic components and heated to the melting temperature (typically around 700° C.). Since the metal and ceramic components contract at different rates upon cooling, large thermal stresses develop at the interface. These stresses can cause the components to delaminate, limiting the application of brazing to metal/ceramic joints that are 1.0 square inch in area or less. Given this inherent difference in thermal contraction and the resulting difficulty, there are no proven methods for bonding large metallic and ceramic components together.
A very important application area of high-temperature bonding of dissimilar materials is in the aerospace and military industries, where bond resistance to high and rapid thermal and mechanical loads is required. In particular, ceramic radomes used in various aerospace vehicles exhibit low weight, high temperature strength, electromagnetic transmissibility, and thermal insulation at boundary layer temperatures over 425° C. The problems associated with the bond durability stem from fundamental differences in the CTE and thermal conductivity. The ceramic has a relatively low CTE, and the metal missile body has a relatively high CTE. In addition, the thermal conductivities are very different too, which actually adds to the problem. Under conditions of rapid temperature increase at the external surfaces of a radome or nose cone, as occurs in flight due to aerodynamic heating effects, it is found that the radome mounting bracket of the metal airframe may become hotter than the ceramic radome because of the differing thermal conductivity characteristics.